821 research outputs found
Incorporation of Boron Atoms on Graphene Grown by Chemical Vapor Deposition Using Triisopropyl Borate as a Single Precursor
We synthesized single-layer graphene from a liquid precursor (triisopropyl borate) using a chemical vapor deposition. Optical microscopy, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy measurements were used for the characterization of the samples. We investigated the effects of the processing temperature and time, as well as the vapor pressure of the precursor. The B1s core-level XPS spectra revealed the presence of boron atoms incorporated into substitutional sites. This result, corroborated by the observed upshift of both G and 2D bands in the Raman spectra, suggests the p-doping of single-layer graphene for the samples prepared at 1000°C and pressures in the range of 75 to 25 mTorr of the precursor vapor. Our results show that, in optimum conditions for single-layer graphene growth, that is, 1000°C and 75 mTorr for 5 minutes, we obtained samples presenting the coexistence of pristine graphene with regions of boron-doped graphene
Nanoindentation unidirectional sliding and lateral force microscopy: evaluation of experimental techniques to measure friction at the nanoscale
CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPERG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DO RIO GRANDE DO SULFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOLateral force microscopy (LFM) is an established technique to assess friction forces at the nanoscale. Nanoindentation followed by unidirectional sliding (NUS) is also used to evaluate friction forces at the micro/nanoscale. However, comparative studies between NUS and LFM evaluating the experimental results at different scales are still missing. In this work, a-C:D/H and a-C:H thin films with different [D]/[C] and [H]/[C] contents were used to analyze the friction forces by NUS and LFM. The results show that the friction behavior assessed by these two techniques in different scales is the same. The correlation between friction forces measured by NUS and LFM depends mainly on a contact area factor that makes invariant the friction force from nanoscale to microscale. Such behavior suggests a similar damping mechanism, probably phonon-coupling phenomena, for the friction force origin.812110CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPERG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DO RIO GRANDE DO SULFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPERG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DO RIO GRANDE DO SULFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO465423/2014-0sem informação2012/10127-
Astrometric calibration and performance of the Dark Energy Camera
We characterize the ability of the Dark Energy Camera (DECam) to perform
relative astrometry across its 500~Mpix, 3 deg^2 science field of view, and
across 4 years of operation. This is done using internal comparisons of ~4x10^7
measurements of high-S/N stellar images obtained in repeat visits to fields of
moderate stellar density, with the telescope dithered to move the sources
around the array. An empirical astrometric model includes terms for: optical
distortions; stray electric fields in the CCD detectors; chromatic terms in the
instrumental and atmospheric optics; shifts in CCD relative positions of up to
~10 um when the DECam temperature cycles; and low-order distortions to each
exposure from changes in atmospheric refraction and telescope alignment. Errors
in this astrometric model are dominated by stochastic variations with typical
amplitudes of 10-30 mas (in a 30 s exposure) and 5-10 arcmin coherence length,
plausibly attributed to Kolmogorov-spectrum atmospheric turbulence. The size of
these atmospheric distortions is not closely related to the seeing. Given an
astrometric reference catalog at density ~0.7 arcmin^{-2}, e.g. from Gaia, the
typical atmospheric distortions can be interpolated to 7 mas RMS accuracy (for
30 s exposures) with 1 arcmin coherence length for residual errors. Remaining
detectable error contributors are 2-4 mas RMS from unmodelled stray electric
fields in the devices, and another 2-4 mas RMS from focal plane shifts between
camera thermal cycles. Thus the astrometric solution for a single DECam
exposure is accurate to 3-6 mas (0.02 pixels, or 300 nm) on the focal plane,
plus the stochastic atmospheric distortion.Comment: Submitted to PAS
Transfer learning for galaxy morphology from one survey to another
© 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.Deep Learning (DL) algorithms for morphological classification of galaxies have proven very successful, mimicking (or even improving) visual classifications. However, these algorithms rely on large training samples of labelled galaxies (typically thousands of them). A key question for using DL classifications in future Big Data surveys is how much of the knowledge acquired from an existing survey can be exported to a new dataset, i.e. if the features learned by the machines are meaningful for different data. We test the performance of DL models, trained with Sloan Digital Sky Survey (SDSS) data, on Dark Energy survey (DES) using images for a sample of 5000 galaxies with a similar redshift distribution to SDSS. Applying the models directly to DES data provides a reasonable global accuracy ( 90%), but small completeness and purity values. A fast domain adaptation step, consisting in a further training with a small DES sample of galaxies (500-300), is enough for obtaining an accuracy > 95% and a significant improvement in the completeness and purity values. This demonstrates that, once trained with a particular dataset, machines can quickly adapt to new instrument characteristics (e.g., PSF, seeing, depth), reducing by almost one order of magnitude the necessary training sample for morphological classification. Redshift evolution effects or significant depth differences are not taken into account in this study.Peer reviewedFinal Accepted Versio
Quasar accretion disk sizes from continuum reverberation mapping in the DES standard-star fields
Measurements of the physical properties of accretion disks in active galactic
nuclei are important for better understanding the growth and evolution of
supermassive black holes. We present the accretion disk sizes of 22 quasars
from continuum reverberation mapping with data from the Dark Energy Survey
(DES) standard star fields and the supernova C fields. We construct continuum
lightcurves with the \textit{griz} photometry that span five seasons of DES
observations. These data sample the time variability of the quasars with a
cadence as short as one day, which corresponds to a rest frame cadence that is
a factor of a few higher than most previous work. We derive time lags between
bands with both JAVELIN and the interpolated cross-correlation function method,
and fit for accretion disk sizes using the JAVELIN Thin Disk model. These new
measurements include disks around black holes with masses as small as
, which have equivalent sizes at 2500\AA \, as small as
light days in the rest frame. We find that most objects have
accretion disk sizes consistent with the prediction of the standard thin disk
model when we take disk variability into account. We have also simulated the
expected yield of accretion disk measurements under various observational
scenarios for the Large Synoptic Survey Telescope Deep Drilling Fields. We find
that the number of disk measurements would increase significantly if the
default cadence is changed from three days to two days or one day.Comment: 33 pages, 24 figure
Photometric redshifts and clustering of emission line galaxies selected jointly by DES and eBOSS
We present the results of the first test plates of the extended Baryon
Oscillation Spectroscopic Survey. This paper focuses on the emission line
galaxies (ELG) population targetted from the Dark Energy Survey (DES)
photometry. We analyse the success rate, efficiency, redshift distribution, and
clustering properties of the targets. From the 9000 spectroscopic redshifts
targetted, 4600 have been selected from the DES photometry. The total success
rate for redshifts between 0.6 and 1.2 is 71\% and 68\% respectively for a
bright and faint, on average more distant, samples including redshifts measured
from a single strong emission line. We find a mean redshift of 0.8 and 0.87,
with 15 and 13\% of unknown redshifts respectively for the bright and faint
samples. In the redshift range 0.6<z<1.2, for the most secure spectroscopic
redshifts, the mean redshift for the bright and faint sample is 0.85 and 0.9
respectively. Star contamination is lower than 2\%. We measure a galaxy bias
averaged on scales of 1 and 10~Mpc/h of 1.72 \pm 0.1 for the bright sample and
of 1.78 \pm 0.12 for the faint sample. The error on the galaxy bias have been
obtained propagating the errors in the correlation function to the fitted
parameters. This redshift evolution for the galaxy bias is in agreement with
theoretical expectations for a galaxy population with MB-5\log h < -21.0. We
note that biasing is derived from the galaxy clustering relative to a model for
the mass fluctuations. We investigate the quality of the DES photometric
redshifts and find that the outlier fraction can be reduced using a comparison
between template fitting and neural network, or using a random forest
algorithm
Target Selection for the Apache Point Observatory Galactic Evolution Experiment (APOGEE)
The Apache Point Observatory Galactic Evolution Experiment (APOGEE) is a
high-resolution infrared spectroscopic survey spanning all Galactic
environments (i.e., bulge, disk, and halo), with the principal goal of
constraining dynamical and chemical evolution models of the Milky Way. APOGEE
takes advantage of the reduced effects of extinction at infrared wavelengths to
observe the inner Galaxy and bulge at an unprecedented level of detail. The
survey's broad spatial and wavelength coverage enables users of APOGEE data to
address numerous Galactic structure and stellar populations issues. In this
paper we describe the APOGEE targeting scheme and document its various target
classes to provide the necessary background and reference information to
analyze samples of APOGEE data with awareness of the imposed selection criteria
and resulting sample properties. APOGEE's primary sample consists of ~100,000
red giant stars, selected to minimize observational biases in age and
metallicity. We present the methodology and considerations that drive the
selection of this sample and evaluate the accuracy, efficiency, and caveats of
the selection and sampling algorithms. We also describe additional target
classes that contribute to the APOGEE sample, including numerous ancillary
science programs, and we outline the targeting data that will be included in
the public data releases.Comment: Accepted to AJ. 31 pages, 11 figure
- …